1. Field of the Invention
The invention relates to a process and apparatus for stripping and regenerating fluidized catalytic cracking catalyst.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425 C-600 C., usually 460 C-560 C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500 C-900 C., usually 600 C-750 C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
Although reactor residence time has continued to decrease, the height of the reactors has not. Although the overall size and height of much of the hardware associated with the FCC unit has decreased, the use of all riser reactors has resulted in catalyst and cracked product being discharged from the riser reactor at a fairly high elevation. This elevation makes it easy for a designer to transport spent catalyst from the riser outlet, to a catalyst stripper at a lower elevation, to a regenerator at a still lower elevation.
The need for a somewhat vertical design, to accommodate the great height of the riser reactor, and the need to have a unit which is compact, efficient, and has a small "footprint" has caused considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article. One modern, compact FCC design is the Kellogg Ultra Orthoflow converter, Model F, which is shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussed above. The compact nature of the design, and the use of a catalyst stripper which is contiguous with and supported by the catalyst regenerator, makes it difficult to expand or modify such units. The catalyst stripper design is basically a good one, which achieves some efficiencies because of its location directly over the bubbling bed regenerator. The stripper can be generously sized, does not have to fit around the riser reactor as in many other units, and the stripper is warmed slightly by its close proximity to the regenerator, which will improve its efficiency slightly.
Although such a unit works well in practice, the stripping of spent catalyst is never as complete as desired by the refiner. In addition, FCC units are being pushed to accept poorer feeds, particularly feeds containing large amounts of resid. These growing demands placed on FCC units and exacerbated four existing problem areas in the regenerator, namely problems with sulfur, steam, temperature and NOx. These problems will each be reviewed in more detail below.
SULFUR
Much of the sulfur in the feed ends up as SOx in the regenerator flue gas. Higher sulfur levels in the feed, combined with a more complete regeneration of the catalyst in the regenerator increases the amount of SOx in the regenerator flue gas. Some attempts have been made to minimize the amount of SOx discharged to the atmosphere through the flue gas by including catalyst additives or agents to react with the SOx in the flue gas. These agents pass with the regenerated catalyst back to the FCC reactor where the reducing atmosphere releases the sulfur compounds as H2S. Suitable agents are described in U.S. Pat. Nos. 4,071,436 and 3,834,031. Use of cerium oxide agent for this purpose is shown in U.S. Pat. No. 4,001,375.
Unfortunately, the conditions in most FCC regenerators are not the best for SOx adsorption. The high temperatures in modern FCC regenerators (up to 870 C. (1600 F.)) impair SOx adsorption. One way to minimize SOx in flue gas is to pass catalyst from the FCC reactor to a long residence time steam stripper, as disclosed in U.S. Pat. No. 4,481,103 to Krambeck et al which is incorporated by reference. This process preferably steam strips spent catalyst at 500-550 C. (932 to 1022 F.), which is beneficial but not sufficient to remove some undesirable sulfur- or hydrogen-containing components.
STEAM
Steam is always present in FCC regenerators although it is known to cause catalyst deactivation. Steam is not intentionally added, but is invariably present, usually as adsorbed or entrained steam from steam stripping of catalyst or as water of combustion formed in the regenerator.
Poor stripping leads to a double dose of steam in the regenerator, first from the adsorbed or entrained steam and second from hydrocarbons left on the catalyst due to poor catalyst stripping. Catalyst passing from an FCC stripper to an FCC regenerator contains hydrogen-containing components, such as coke or unstripped hydrocarbons adhering thereto. This hydrogen burns in the regenerator to form water and cause hydrothermal degradation.
Steaming of catalyst becomes more of a problem as regenerators get hotter. Higher temperatures greatly accelerate the deactivating effects of steam.
TEMPERATURE
Regenerators are operating at higher and higher temperatures. This is because most FCC units are heat balanced, that is, the endothermic heat of the cracking reaction is supplied by burning the coke deposited on the catalyst. With heavier feeds, more coke is deposited on the catalyst than is needed for the cracking reaction. The regenerator gets hotter, and the extra heat is rejected as high temperature flue gas. Many refiners severely limit the amount of resid or similar high CCR feeds to that amount which can be tolerated by the unit. High temperatures are a problem for the metallurgy of many units, but more importantly, are a problem for the catalyst. In the regenerator, the burning of coke and unstripped hydrocarbons leads to much higher surface temperatures on the catalyst than the measured dense bed or dilute phase temperature. This is discussed by Occelli et al in Dual-Function Cracking Catalyst Mixtures, Ch. 12, Fluid Catalytic Cracking, ACS Symposium Series 375, American Chemical Society, Washington, D.C., 1988.
Some regenerator temperature control is possible by adjusting the CO/CO2 ratio produced in the regenerator. Burning coke partially to CO produces less heat than complete combustion to CO2. However, in some cases, this control is insufficient, and also leads to increased CO emissions, which can be a problem unless a CO boiler is present.
U.S. Pat. No. 4,353,812 to Lomas et al, which is incorporated by reference, discloses cooling catalyst from a regenerator by passing it through the shell side of a heat-exchanger with a cooling medium through the tube side. The cooled catalyst is recycled to the regeneration zone. The Kellogg H.O.C. regenerator has a catalyst cooler connected to the dense bed of the regenerator. These approaches remove heat from the regenerator, but will not prevent poorly, or even well, stripped catalyst from experiencing very high surface or localized temperatures in the regenerator.
The prior art also used dense or dilute phase regenerated fluid catalyst heat removal zones or heat-exchangers that are remote from, and external to, the regenerator vessel to cool hot regenerated catalyst for return to the regenerator. Examples of such processes are found in U.S. Pat. Nos. 2,970,117 to Harper; 2,873,175 to Owens; 2,862,798 to McKinney; 2,596,748 to Watson et al; 2,515,156 to Jahnig et al; 2,492,948 to Berger; and 2,506,123 to Watson.
NOX
Burning of nitrogenous compounds in FCC regenerators has long led to creation of minor amounts of NOx, some of which were emitted with the regenerator flue gas. Usually these emissions were not much of a problem because of relatively low temperature, a relatively reducing atmosphere from partial combustion of CO and the absence of catalytic metals like Pt in the regenerator which increase NOx production.
Many FCC units now operate at higher temperatures, with a more oxidizing atmosphere, and use CO combustion promoters such as Pt. These changes in regenerator operation reduce CO emissions, but usually increase nitrogen oxides (NOx) in the regenerator flue gas. It is difficult in a catalyst regenerator to completely burn coke and CO in the regenerator without increasing the NOx content of the regenerator flue gas, so NOx emissions are now frequently a problem. These problems are more severe in bubbling bed regenerators, because of relatively poor catalyst circulation (large stagnant regions in the dense bed) and the presence of large bubbles of regeneration gas which leads to localized high concentrations of oxygen, which increases NOx emissions.
Recent catalyst patents include U.S. Pat. No. 4,300,997 and its division U.S. Pat. No. 4,350,615, both directed to the use of Pd-Ru CO-combustion promoter. The bi-metallic CO combustion promoter is reported to do an adequate job of converting CO to CO2, while minimizing the formation of NOx.
U.S. Pat. No. 4,199,435 suggests steam treating conventional metallic CO combustion promoter to decrease NOx formation without impairing too much the CO combustion activity of the promoter.
Process modifications are suggested in U.S. Pat. No. 4,413,573 and U.S. Pat. No. 4,325,833 directed to two-and three-stage FCC regenerators, which reduce NOx emissions.
U.S. Pat. No. 4,313,848 teaches countercurrent regeneration of spent FCC catalyst, without backmixing, to minimize NOx emissions.
While such process modifications may be useful for new construction they cannot be easily added to existing units, especially not to compact regenerator/stripper designs such as the Kellogg H.O.C. regenerator.
U.S. Pat. No. 4,309,309 teaches the addition of a vaporizable fuel to the upper portion of a FCC regenerator to minimize NOx emissions. Oxides of nitrogen formed in the lower portion of the regenerator are reduced in the reducing atmosphere generated by burning fuel in the upper portion of the regenerator.
U.S. Pat. No. 4,235,704 suggests that too much CO combustion promoter causes NOx formation, and calls for monitoring the NOx content of the flue gases, and adjusting the concentration of CO combustion promoter in the regenerator based on the amount of NOx in the flue gas.
The approach taken in U.S. Pat. No. 4,542,114 is to minimize the volume of flue gas by using oxygen rather than air in the FCC regenerator, with consequent reduction in the amount of flue gas produced.
All the catalyst and process patents discussed above, directed to reducing NOx emissions, from U.S. Pat. No. 4,300,997 to U.S. Pat. No. 4,542,114, are incorporated herein by reference.
The reduction in NOx emissions achieved by the above approaches helps some but still may fail to meet the ever more stringent NOx emissions limits set by local governing bodies. Much of the NOx formed is not the result of combustion of N2 within the FCC regenerator, but rather combustion of nitrogen-containing compounds in the coke entering the FCC regenerator.
Unfortunately, the trend to heavier feeds usually means that the amount of nitrogen compounds on the coke will increase so NOx emissions will increase. Higher regenerator temperatures also tend to increase NOx emissions. It would be beneficial, in existing refineries, to have a way to reduce NOx emissions so that heavier feeds, and environmental concerns, can be accommodated.
We realized that a better catalyst stripper design is needed. A better stripper would attack most of the problems in the regenerator at their source, namely poor stripping. Better stripping would permit increased recovery of valuable, strippable hydrocarbons and remove more hydrogen from spent catalyst to minimize hydrothermal degradation in the regenerator. It would also remove more sulfur-containing compounds from spent catalyst prior to regeneration to minimize SOx in the regenerator flue gas and would help reduce regenerator temperature by reducing the amount of material burned in it. The problems were obvious, but a solution to these problems, which could be incorporated into existing FCC regenerators, especially compact designs, was not.
We reviewed the work that others had done on improving stripping, and found nothing directly applicable to the special problems of better stripping in FCC units where the stripper was so closely associated with, and supported by, a bubbling dense bed regenerator. The improvements in stripping in FCC units where the stripper was remote from the regenerator were not directly applicable. Thus we could not readily use the hot stripper design of U.S. Pat. No. 4,820,404 (Owen), which is easiest to implement in units where the regenerator is at a higher elevation that the catalyst stripper. For similar reasons we could not use the multi-stage hot strippers of U.S. Pat. No. 4,789,458 (Haddad, Owen, Schatz).
We discovered a way to achieve high temperature stripping of coked FCC catalyst which could be readily retrofitted into strippers operating above and supported by bubbling dense bed regenerators. We discovered a way to make the vices of the existing design, it compactness and proximity to the bubbling bed regenerator, virtues which allowed us to achieve an unexpectedly effective hot stripping design. We found a way to improve stripping, increase the yield of valuable liquid product, reduce the load placed on the catalyst regenerator, minimize SOx and NOx emissions and permit the unit to process more difficult feeds. Regenerator temperatures can be reduced somewhat, and the hydrothermal deactivation of catalyst in the regenerator reduced. All this could be accomplished generally within the confines of existing equipment, and with significantly less regenerated catalyst circulation to the stripper (for direct contact heat exchange) than would be expected based on other hot stripper designs.